Electronic Structure of ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$

We have analyzed the unusual electronic structure of ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ combining ab initio and model Hamiltonian approaches. Our results indicate that there are strong enhancements of the intra-atomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss the possibility of a negative effective Coulomb correlation strength ( $U_{\mathrm{eff}}$) at the Mo site due to these renormalized interaction strengths.